1. Field of the Invention
This invention relates to a method for purifying fine particulate silica such as fumed silica by eliminating halide therefrom.
2. Prior Art
Fumed silica is widely used in the art as a filler for silicone elastomers and various other resins. In general, fumed silica is prepared by pyrogenic hydrolysis of silicon-containing halides. Since fumed silica has high surface activity and a large specific surface area, it has adsorbed thereon a substantial amount of hydrogen halide resulting from pyrogenic hydrolysis as a by-product. Fumed silica bears halogen in a chemically bonded state if the reaction is incomplete. It is undesirable to use fumed silica bearing a noticeable amount of halogen as a filler without treatment. Usually, such fumed silica is used after deacidifying treatment is carried out for eliminating the halide.
Conventionally, fumed silica is deacidified by using an endless steel belt or a rotary column with transfer means for bringing fumed silica in contact with wet air at 200.degree. to 500.degree. C. as disclosed in Japanese Patent Publication (JP-B) No. 46274/1972. Although deacidification can be done at relatively low temperatures, the apparatus includes mechanical movable parts which tend to fail during use in a hot acidic corrosive atmosphere.
Another known deacidifying method is countercurrent contact between fumed silica and wet air as disclosed in JP-B 13832/1973. Although this method avoids to place mechanical movable parts in a hot acidic corrosive atmosphere and allows for use of a relatively simple apparatus, the method is less efficient in that separation of coarse particles cannot be accomplished concurrently with the deacidification. The treating temperature is relatively high. JP-B 13832/1973 describes a comparative example using a parallel flow fluidized layer. It is described that this parallel flow fluidized layer is at a temperature of 600.degree. to 800.degree. C. Use of such high temperature is impractical because the material of the apparatus and the heat source are limited.